A system for classifying Giardia lamblia isolates was developed. Isolates fell into at least three major groups some of which differed greatly indicating these groups separated tens of millions of years ago. An adult mouse model of infection was developed. After an intense acute infection, trophozoites survive in very low numbers for months. A LIN-like Zn binding domain was identified in most variant- specific surface proteins and shown to bind Zn as well as other cationic metals. The biological significance of this is not known but it is possible that trophozoites bind intestinal Zn thereby preventing absorption and causing clinical or subclinical Zn deficient.